1. Field of the Invention
The present invention relates to an apparatus for producing a plastic sheet and a process for producing a plastic sheet using the same. More particularly, the present invention relates to an apparatus for producing a laminate sheet having one or more layers or a sheet of a thick single layer using a polyolefin molding material comprising an inorganic filler and a polyolefin resin as the substrate material without warp, with an excellent flatness of the surface, in a reduced cooling time and with excellent productivity and a process for producing the above laminate sheet or the above sheet of a thick single layer using this apparatus.
2. Description of Related Art
Materials heretofore used as decorative panels at the face of furnitures and cabinets of kitchen products have a structure in which decorative sheets having printed patterns such as wood patterns are laminated to a wooden material, an inorganic material or a metal material such as a steel plate with an adhesive.
Sheets of vinyl chloride resins are generally used as the decorative sheets used for such decorative panels due to a suitable degree of flexibility and excellent workability in V-cutting and lapping. However, the sheets of vinyl chloride resins have drawbacks in that durability such as resistance to wear, scratches and dents and the property for cleaning are inferior, that environmental problems are caused due to formation of toxic substances such as chlorine and dioxine during incineration and that plasticizers contained therein tend to bleed to cause inferior appearance.
Therefore, development of a decorative sheet or a decorative panel having excellent durability, an excellent property for cleaning and excellent workability such as workability in V-cutting without using a vinyl chloride resin has been desired.
As decorative materials for buildings, wood and vinyl chloride resins have been used mainly. However, it is inevitable that the use of wood is restricted to protect forests. The vinyl chloride resins have the drawbacks described above. Therefore, development of decorative materials for buildings which do not cause environmental problems without using wood or a vinyl chloride resin has been desired.
As the decorative material without the use of wood or a vinyl chloride resin, composite plates in which a synthetic resin such as a polyolefin resin is used as a core material and two sheets of a metal such as aluminum, iron and stainless steel are laminated to both faces of the core material have been-developed and are actually used. These composite plates show excellent light weight, workability and impact strength and are used as interior and outer decorative members.
Sheets of a thick single layer which are made of polyolefin resins and have a thickness of 5 to 15 mm are also attracting attention as various products such as chopping boards and molding boxes for concrete.
When decorative sheets, decorative panels, decorative materials for buildings and sheets of a thick single layer are produced, it is very important that excellent products without warp are produced with excellent productivity from the standpoint of the production cost and commercial value.
When a laminate sheet prepared at a high temperature by melt adhesion of layers under heating to a substrate sheet of a polyolefin resin or a sheet of a thick single layer prepared at a high temperature from a polyolefin resin is continuously cooled, the sheet is generally introduced into a cooling apparatus equipped with a plurality of cooling rolls through which cooling water is passed for cooling. Since rapid cooling is not preferable, the temperature of cooling water is adjusted in a manner such that the temperature decreases stepwise from cooling rolls at upstream positions to cooling rolls at downstream positions and the temperature of the sheet is reduced approximately to the room temperature at the outlet of the apparatus.
However, the above method of cooling has drawbacks in that productivity is poor due to a long time required for cooling and that warp of the produced sheet tends to take place.
When many types of decorative sheets or decorative panels are produced in small amounts, the laminates are generally prepared by melt adhesion under heating or dry lamination in accordance with a batch pressing process and the prepared products are cooled just by being left standing. This process has a drawback in that it takes an excessively long time for cooling to cause poor productivity.
An object of the present invention is to provide an apparatus for producing a plastic sheet which can produce various types of laminate sheets and sheets of a thick single layer having various thickness such as decorative sheets, decorative panels and decorative building materials, which do not form toxic substances such as chlorine gas during incineration and do not cause environmental problems, without warp, with an excellent flat surface, in a reduced cooling time and with excellent productivity and to provide a process for producing the above laminate sheets and the sheets of a thick single layer using the above apparatus with excellent productivity.
As the result of extensive studies by the present inventors to achieve the above object, it was found that an apparatus comprising at least a mechanism for preparing a laminate sheet or a sheet of a thick single layer using a polyolefin molding material comprising an inorganic filler and a polyolefin resin as the substrate material and a cooling mechanism having a specific construction can be advantageously used to achieved the above object and that, using this apparatus, the above laminate sheet and the above sheet of a thick single layer can be produced without warp, with an excellent flat surface and with excellent productivity. The present invention has been completed on the basis of the knowledge.
The present invention provides:
(1) An apparatus for producing a plastic sheet which comprises at least (A) a mechanism for preparing a plastic sheet which is a laminate sheet comprising one or more laminated layers or a sheet of a thick single layer at a high temperature using a polyolefin molding material comprising an inorganic filler and a polyolefin resin as a substrate material and (B) a cooling mechanism for cooling the plastic sheet which is prepared by mechanism (A) and at a high temperature, wherein cooling mechanism (B) comprises a cooling apparatus which comprises a lower endless metal belt and an upper endless metal belt disposed on top of the lower belt, each belt being disposed with tension along a loop surrounding two rolls, movable along the loop and cooled by direct contact with cooling water, so that the plastic sheet at a high temperature is held between the belts and cooled at both sides;
(2) An apparatus described in (1), wherein cooling mechanism (B) comprises a plurality of cooling apparatuses which are arranged in series and each comprises a lower endless metal belt and an upper endless metal belt disposed on top of the lower belt, each belt being disposed with tension along a loop surrounding two rolls, movable along the loop and cooled by direct contact with cooling water;
(3) An apparatus described in (2), wherein cooling mechanism (B) comprise two cooling apparatuses arranged in series;
(4) An apparatus described in (1), wherein cooling mechanism (B) comprises one cooling apparatus which comprises a lower endless metal belt and an upper endless metal belt disposed on top of the lower belt, each belt being disposed with tension along a loop surrounding two rolls, movable along the loop and cooled by direct contact with cooling water, wherein each belt is equipped with a mechanism for cooling the plastic sheet at a high temperature in a manner such that a front portion of the belt which is a portion closer to a front roll and a rear portion of the belt which is a portion closer to a rear roll are cooled separately by direct contact with water at different temperatures while the belt is in movement;
(5) A process for producing a plastic sheet comprising, using an apparatus described in (1), (a) preparing a plastic sheet which is a laminate sheet comprising one or more laminated layers or a sheet of a thick single layer at a high temperature using a polyolefin molding material comprising an inorganic filler and a polyolefin resin as a substrate material and (b) cooling the plastic sheet which is prepared in step (a) and at a high temperature, wherein the plastic sheet is cooled by passing between a lower endless metal belt and an upper endless metal belt disposed on top of the lower belt, each belt being disposed with tension along a loop surrounding two rolls, moving along the loop and being cooled by direct contact with cooling water;
(6) A process described in (5), which comprises cooling the plastic sheet at a high temperature using a plurality of apparatuses described in (2) in a plurality of stages, wherein, in step (b), a temperature of the lower endless metal belt and the upper endless metal belt disposed on top of the lower belt in each cooling apparatus is adjusted while the belts are in movement along the loop in a manner such that a temperature of the belts in a cooling apparatus at a downstream position is successively lower than a temperature of the belts in another apparatus at an upstream position, the belts in a cooling apparatus at a most upstream position are directly cooled with water at a temperature of 10 to 40xc2x0 C. and the belts in a cooling apparatus at a most downstream position is directly cooled with water at a temperature of 4 to 10xc2x0 C.;
(7) A process described in (5), which comprises cooling the hot plastic sheet using an apparatus described in (4), wherein, in step (b), each of the lower endless metal belt and the upper endless metal belt disposed on top of the lower belt in the cooling apparatus is cooled while the belt is in movement along the loop in a manner such that a portion of the belt at an upstream position is directly cooled with water at a temperature of 10 to 40xc2x0 C. and a portion of the belt at a downstream position is directly cooled with water at a temperature of 4 to 10xc2x0 C.;
(8) A process for producing a plastic sheet described in any of (5), (6 ) and
(7), wherein the polyolefin molding material comprises 20 to 80% by weight of an inorganic filler; and
(9). A process for producing a plastic sheet described in (8), wherein the inorganic filler is at least one filler selected from the group consisting of calcium carbonate, aluminum hydroxide and magnesium hydroxide.